The invention relates to a particle-optical apparatus for changing trajectories of charged particles of a beam of particles. Furthermore, the invention relates to an illumination apparatus and a projection system comprising such a particle-optical apparatus as well as a method for device manufacture. Such method comprises a photolithographic step in which the particle-optical apparatus is employed. In particular, the particle-optical apparatus is provided for use in an projection electron-beam lithographic system as well as for use in a method for device manufacture by means of projection electron-beam lithography.
The so-called SCALPEL method (Scattering with Angular Limitation in Projection Electron-beam Lithography) is known as a method which employs a beam of electrons for imaging and exposing a radiation sensitive layer. This method is described in the white book xe2x80x9cSCALPEL: A Projection Electron-Beam Approach to Sub-Optical Lithographyxe2x80x9d, Technology Review, December 1999, by J. A. Liddle, Lloyd R. Harriott, A. E. Novembre and W. K. Waskiewicz, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, N.J. 07974, USA. The entire disclosure of said document is incorporated in this description by reference. Furthermore, U.S. Pat. Nos. 5,079,112, 5,130,213, 5,260,151, 5,376,505, 5,258,246, 5,316,879 as well as European patent applications nos. 0,953,876 A2 and 0,969,326 A2 relate to the SCALPEL method. The entire disclosures of the above-mentioned patent documents are likewise incorporated in this description by reference.
A conventional projection lithographic system is used, for example, for the manufacture of a semiconductor device. Here, the structures to be formed on a semiconductor wafer are defined in a mask, the mask is illuminated by a beam of electrons and the structures defined on the mask are imaged onto the semiconductor wafer. The semiconductor wafer is provided with a radiation sensitive layer. After having been exposed by the electron beam, the radiation sensitive layer as well as the semiconductor wafer are subjected to further steps for forming the structures in the wafer material.
FIG. 1 schematically shows an illumination apparatus for illuminating a mask 3 with charged particles. The charged particles are electrons which are emitted by an electron source 5 in a beam direction 7. The particle beam emitted by the source 5 exhibits little divergence which, for reasons of illustration, is, however, shown in FIG. 1 enlarged in size. A maximum angle xcex1 of the electrons with respect to the beam direction 7 is about 5 mrad.
The source is imaged by a first electron-optical focusing lens 9 into a front focal plane 11 of a second electron-optical focusing lens 13. The focusing lens 13 acts to shape the electrons divergently traversing the focal plane 11 such that a substantially parallel particle beam 15 with extended beam cross-section is formed in order to illuminate a field 17 on the mask 3 of a size of about 1 mm transverse to the beam direction 7.
The maximum illumination aperture which is attainable with this type of illumination is determined by the spatial dimension h of the source 5 transverse to the beam direction 7 as well as by the focal length f1 of the lens 13. The maximum angle xcex2 of the particles with respect to the beam direction 7, when the same impinge on the mask 3, is determined by   β  =            h              2        ·                  f          1                      .  
For small dimensions of the source 3 (FIG. 1 shows a dot-shaped source), the illumination aperture is thus low. However, a high illumination aperture is desirable in order to be able to transfer also small structures defined on the mask to the wafer with precision.
It is conceivable to increase the spatial dimension of the source transverse to the beam direction in order to increase the illumination aperture. However, it is problematic for sources of charged particles to increase the source dimension if the field illuminated on the mask is to be uniformly illuminated as well.
It is an object of the present invention to provide a particle-optical apparatus which contributes to the increase of an illumination aperture in a particle-optical illumination system.
Moreover, it is an object of the present invention to propose a particle-optical apparatus for changing trajectories of charged particles of a particle beam. In this respect, it is, in particular, an object of the invention to propose a particle-optical apparatus which selectively changes the trajectories of the charged particle, i.e., which acts only on trajectories of specific charged particles and not uniformly on the trajectories of all particles of a particle beam.
Furthermore, it is an object of the present invention to propose an illumination apparatus for illuminating a field which is to be illuminated and is spatially extended transverse to the beam direction with a comparatively high illumination aperture or/and comparatively uniformly.
Moreover, it is an object to propose a projection system, the illumination apparatus of which exhibits the above-mentioned advantages. It is a still further object of the invention to propose a method for manufacturing in particular miniaturized devices which enables the devices to be manufactured with increased precision.
To this end, the invention is based on the following consideration:
In an imaging illumination system, as it has been described above with reference to FIG. 1 by way of example, the light-transmitting value or emittance is a conservative quantity. This quantity is defined as the product of the square root of the illuminated area and the illumination divergence (numerical aperture). In an imaging illumination system, an increase of the illumination divergence is thus not achievable without decreasing the illuminated area. Therefore, the invention is based on the idea to develop a particle-optical apparatus which does not act as an imaging system but changes the trajectories of the charged particles traversing the particle-optical apparatus in a different way. The trajectories of different groups of particles are to be changed differently such that, all in all, an increase of the light transmitting value or emittance of the beam passing through the apparatus is achieved.
In particular, the invention proposes a particle-optical apparatus comprising two cylindrical electrode arrangements which are fitted into one another, said electrode arrangements being disposed relative to a particle beam entering the apparatus such that the beam direction is oriented approximately parallel to the direction of extension of at least one of the cylindrical electrodes. Moreover, an inner one of the two electrode arrangements is of such a length and has such a diameter that trajectories of at least those particles which enter the apparatus at an angle with respect to the beam axis which is larger than a minimum angle traverse the inner electrode arrangement radially with respect to the beam direction. To this end, the inner electrode arrangement is at least partially transparent for the charged particles. There is an electric potential difference between the inner electrode arrangement and the outer electrode arrangement such that a kinetic component of the particles traversing the inner electrode arrangement is reversed, said kinetic component being oriented transversely to the beam direction.
The inner and outer electrode arrangements together act like a cylindrical, internally mirrored tube which encloses the particle beam and reflects particles which want to escape from the interior of the cylinder back into the same.
For a group of particles of the particle beam which enter the apparatus with little divergence, the apparatus is preferably not effective, that is, this group of particles traverses the apparatus straightly, so that an observer positioned at the exit side of the apparatus perceives these particles as emerging from the particle source.
For another group of particles with increased divergence, the apparatus is preferably effective such that the particles are reflected once by the reflecting tube. The observer perceives this group of particles as emerging from a spatially distributed source which appears to be disposed beside the actual source.
For a still further group of particles with a still higher divergence, the apparatus is effective such that these particles are reflected twice or more by the reflecting tube so that the observer perceives this group of particles to emerge from further spatially distributed sources which appear to be spaced apart by a still greater distance from the actual source.
Accordingly, the effect of the apparatus of the invention is such that even a small radiation source is perceived by the observer as a radiation source which appears to have a substantially increased radiation emitting area.
If such a particle-optical apparatus is used in an illumination system, it contributes to an apparent increase of the spatial dimension of the radiation source transverse to the beam direction. This results into an increase of the light-transmitting value or emittance of the illumination system. For this reason, the apparatus of the invention also enables the illumination aperture to be increased for an illuminated field which is extended transverse to the beam direction.
Due to the potential difference between the inner electrode arrangement and the outer electrode arrangement, there is provided a space between these two electrode arrangements with an electric field therein which renders it possible to reverse the transversal kinetic component of the particles which enter said space. As this field is limited to the space between the inner electrode arrangement and the outer electrode arrangement, the charged particles must be enabled to enter this space. For this reason, the inner electrode arrangement is at least partially transparent for these particles. Preferably, this property of the inner electrode arrangement, namely partial transparency, is achieved in that the inner electrode arrangement is divided into a plurality of sub-electrodes which are spaced apart from one another. Preferably, the individual sub-electrodes are on a common equal electric potential, and a material-free space is provided between the sub-electrodes so that the particles pass through two adjacent sub-electrodes and can enter the space between the inner electrode arrangement and the outer electrode arrangement which provides the reflecting electric field. However, it is also possible that particles impinge directly on the sub-electrodes and thus cannot enter the space between inner and outer electrode.
In order to obtain a transparency as high as possible for the charged particles, the sub-electrodes preferably extend substantially parallel to the longitudinal axis of the apparatus or/and substantially parallel to the main direction of the particle beam entering the apparatus.
If the above-described particle-optical apparatus is employed in an illumination apparatus for illuminating an object, it acts there as an emittance changing apparatus which is preferably disposed between a particle source and the object plane. Preferably, such an illumination apparatus also comprises an imaging condenser system which is disposed between the emittance changing apparatus and the object and directs the particles emerging from the emittance changing apparatus to the object.
Preferably, the particles emitted by the particle source pass directly into the emittance changing apparatus. It is, however, also preferred to provide an optical system between the particle source and the emittance changing apparatus for producing an image of the source between the actual source and an entrance cross-section of the emittance changing apparatus.
Preferably, the condenser system comprises a focusing lens which images the source itself or the image of the source into an intermediate plane disposed between the emittance changing apparatus and the object plane. As a result, preferably several images of the source itself are produced in the intermediate plane if the first focusing lens images the source in the intermediate plane, and several images of the image of the source are produced if the first focusing lens images an image of the source in the intermediate plane. This plurality of images of the source or of the images of the source images are distributed in the intermediate plane, in particular, adjacent to one another.
However, it is also preferred that the first focusing lens does not produce an exact image of the source or of the source image in the intermediate plane. In this case, it is essential for the first focusing lens to direct the particles which have passed through the emittance changing apparatus in such a way through the intermediate plane that they are spaced apart in the intermediate plane from the longitudinal axis of the apparatus or the beam center by a distance which increases the more often the trajectory of a particle has been changed by the emittance changing apparatus. As compared to a situation in which the emittance changing apparatus is not disposed in the beam path, there is thus provided in the intermediate plane an extended illuminated area or an extended area which is traversed by particles. The particles traversing this extended area are directed to the object plane preferably by means of a second focusing lens such that in the object plane a field is illuminated which has a dimension in a direction transverse to the beam direction which is smaller than the dimension of the illuminated area in the intermediate plane. However, as compared to the situation in which the emittance changing apparatus is not disposed in the beam path, the illumination of this area in the object plane is then effected with an increased numerical aperture.
The invention also provides for a method for device manufacture, such devices being preferably highly miniaturized devices, such as micro-mechanical structures or integrated circuits. As far as integrated circuits are concerned, a mask includes a circuit pattern which corresponds to a single layer of the circuit to be formed on a suitable substrate, for example, a silicon wafer. In order to image the pattern onto a target area, also referred to as die, of the substrate, the latter is first covered with a radiation sensitive layer, also referred to as resist. Subsequently, the radiation sensitive layer is exposed or irradiated in that the pattern of the mask is imaged by means of charged particles, for example, electrons or ions, onto the radiation sensitive layer. The radiation sensitive layer is then developed and either the irradiated or exposed or the non-irradiated or unexposed regions of the irradiated layer are removed. The remaining structure of the radiation sensitive layer is then used as a mask, for example, in an etching step, an ion implantation step, a material deposition step or the like.
According to the invention, it is provided for that the mask and the structure which is defined on the mask and which is to be imaged onto the substrate are illuminated by the above-described illumination apparatus in a photolithographic step of the method.
Exemplary embodiments of the invention will be described below in further detail with reference to the accompanying drawings, wherein